This invention relates generally to a document reproduction apparatus and, more particularly, to a flash illumination imaging system designed to reduce the distortion effects resulting from original documents having creases and curls.
Present day high-speed copiers and duplicators such as the Xerox Model 1075 and Xerox Model 1090 utilize flash exposure of an entire document to create an image of the document on a photoreceptor belt. These high-speed copiers typically employ some type of automatic document handler to move documents to and from the exposure location on a platen. They also utilize a mirror-less optical system which incorporates a wide angle lens to project a full-frame image of an illuminated document onto a flat photoreceptor surface. A particular problem encountered with these types of systems is the inability to faithfully reproduce those portions of an original document which are not perfectly flat, e.g., have been subject to creasing, wrinkling or curling. This is caused by the fact that the lens has a larger field angle than an optical system using a folded mirror optical path. While the lens has sufficient depth of focus to compensate for curling; straight lines in curled or wrinkled areas appear curved (distorted) at the image plane. As shown in FIG. 1, document 4, placed on platen 6, has; as an example, a curl at its edge. With a flash illumination system, a chief ray through point P on the document edge is imaged as though its point was at P1 and projected by a lens 8 as a focused, distorted image at an imaging plane. The desired position for point P is at point P", the position of the document edge in a curl-free condition. (For simplicity the shift of the chief ray as it passes through the platen is not shown). The amount of distortion of the imaged point is proportional to the angle .theta.; e.g. large angles of .theta. cause more image distortion. The desired condition is .theta.=0. A further description of this distortion effect is found on pages 39-40 of "Lenses in Photography" by Rudolf Kingslake, 1951, Case-Hoyt Corporation, Rochester.
For open platen copying, an operator can usually manually flatten the document during the copying mode. Since this is not possible during a document handling mode, several expedients are known in the art to correct or compensate for this problem. In U.S. Pat. No. 4,335,954, a document transported to the top of a platen is maintained in a flat orientation by a clamp mechanism which is lowered to flatten the document during the exposure. In U.S. Pat. No. 4,043,665 a document handling system is disclosed which includes a vacuum belt spaced a small distance above the platen. These systems compensate to some degree for minor document wrinkling, but they do not entirely solve the problem of the more severe wrinkles and curls. It is also not appropriate for use with applications requiring book copying.
The present invention is directed towards a mechanism for reducing or eliminating distortion caused by imaging non-planar documents located in an object plane. The invention includes modifying the document platen in such a way as to add positive optical power so as to bend the imaged rays in order to reduce the distortion effects of non-planar (wrinkled or curled) portions of the document. More particularly, the invention is directed towards a transparent platen located in an object plane; means for positioning an original document on the surface of said platen; means for projecting an image of said document onto a photosensitive surface; and a positive lens element positioned between said platen and said projection lens said lens adapted to compensate for distortion caused by imaging of non-planar portions of said documents.